Philip Austin

The atmospheric convective boundary layer has been studied for over thirty years in order to understand the dynamics and scaling behaviour of its growth by entrainment. This enables prediction of its entrainment rate and entrainment zone depth, and so parameterizations thereof for use in global circulation models.Fundamentals, such as the dependence of the entrainment rate and entrainment zone depth on the convective Richardson number, have been established but there is still unresolved discussion about the form of these relationships. Details regarding the structure of the entrainment zone continue to emerge. The variety of convective boundary layer height and entrainment zone depth definitions adds further complexity. The study described in this thesis aims to join this ongoing discussion.A dry, shear-free, idealized convective boundary layer in the absence of large scale winds was modeled using a large eddy simulation. The use of ten ensemble cases enabled calculation of true ensemble averages and potential temperature fluctuations as well as providing smooth average profiles. A range of convective Richardson numbers was achieved by varying the two principle external parameters: surface vertical heat flux and stable upper lapse rate.The gradient method for determining local convective boundary layer height was found to be unreliable so a multi-linear regression method was used instead. Distributions of the local heights thus determined were found to narrow with increased upper stability. Height and entrainment zone depth were then defined based on the ensemble and horizontally averaged potential temperature profile. The resulting relationships of entrainment rate and entrainment zone depth to Richardson number showed behaviour in general agreement with theory and the results of other studies. The potential temperature gradient in the upper convective boundary layer and entrainment zone was seen to depend on the upper lapse rate, as was the positive downward moving temperature fluctuations at the \acs{CBL} top. Overall, once the surface heat flux was accounted for by applying the \acs{CBL} height as a scale, the upper lapse rate emerged as the dominant parameter influencing scaled entrainment zone depth, and potential temperature variance in the entrainment zone and upper convective boundary layer.

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Clouds, especially low clouds, are key to our ability to understand and predict climate. They are an important component of the physical climate system and contribute significantly to the difference in climate projections by General Circulation Models (GCMs). Cloud predictors, such as atmospheric stability and large-scale circulation, are often used in model parametrizations. This thesis evaluates the performance of the latest Canadian atmospheric GCM (CanAM-4.1), in particular with respect to its cloud simulation. Its output is compared to observations, re-analyses, and its predecessor (AGCM-3). The analysis focuses on low clouds in the tropical band (30 degree South to 30 degree North) over the ocean. Results show that CanAM-4.1 systematically performs better than AGCM-3 (when compared to observations). Variability between observational datasets is also shown to be much smaller than variability between observations and models (or re-analyses). A model-to-satellite approach is used, i.e. the CFMIP Observation Simulator Package (COSP), and reduces observations-CanAM-4.1 differences in low cloud fractions. Results are not as unambiguous for high clouds. Three cloud regimes (stratiform, convectiform, and storm track) are well reproduced by all datasets, i.e. CanAM-4.1, AGCM-3, the ECMWF Interim Re-Analysis (ERA-Interim), and the ECMWF 40 years Re-Analysis (ERA-40). Conditional sampling of low cloud fractions as a function of the Lower Tropospheric Stability (LTS), Estimated Inversion Strength (EIS), and vertical velocity at the 500 hPa level (ω₅₀₀) show good agreement with observations. Overall, conclusions are not sensitive to using EIS rather than LTS, except for the storm track regime. In comparison to observations from the International Satellite Cloud Climatology Project (ISCCP), CanAM-4.1, AGCM-3, and ERA-40 underestimate the low cloud fraction in stratiform regimes. ERA-Interim is shown to reproduce particularly well low cloud regimes and the relationship between large-scale circulation and stability.

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